Apparatus for welding a conductor through cold flowable insulation

ABSTRACT

A CONDUCTOR, SURROUNDED WITH COLD FLOWABLE INSULATION, IS WELDED TO A TERMINAL PIN, E.G., BY ENGAGING THE CONDUCTOR AND THE PIN BETWEEN A PAIR UNHEATED WELDING ELECTRODES. THE ELECTRODES ARE URGED TOGETHER WITH A FORCE WHICH REACHES A VALUE GREATER THAN A DESIRED WELDING FORCE VALUE AND IS SUFFICIENT TO CAUSE THE INSULATION TO UNDERGO COLD FLOW OUT OF A PATH BETWEEN THE ELECTODES VIA THE CONDUCTOR AND THE PIN, THEREBY TO RENDER THE PATH CONDUCTIVE. THE FORCE APPLIED TO THE ELECTRODES IS THEN DECREASED TO THE DESIRED WELDING FORCE VALUE AND A PULSE OF WELDING CURRENT IS PASSED ALONG THE CONDUCTIVE PATH WHEN THE DESIRED FORCED VALUE IS ACHIEVED. THE CURRENT PULSE IS APPLIED AS THE FORCE APPLIED TO THE ELECTRODES DECEASES AND IN COMPLETED, AND THE MOLTEN METAL FORMED BY THE CURRENT PULSE IS SOLIDIFIED, BEFORE SUCH FORCE IS COMPLETELEY REMOVED. THE CONDUCTOR IS SUPPLIED THROUGH A PASSAGE FORMED IN ONE OF THE ELECTRODES.

Dec. 12, 1972 J. F. WEATHERMAN ET AL APPARATUS FOR WELDING A CONDUCTORTHROUGH COLD FLOWABLE INSULATION Original Filed July 28, 1969 '56 l ii l1 5/ 4 49 32 Ii.

4 Sheets-Sheet 1 37 52 60 lllh'i Dec. 12, 1972 WEATHERMAN EI'AL3,705,970

APPARATUS FOR WELDING A CONDUCTOR THROUGH COLD FLOWABLE INSULATIONOriginal Filed July 28, 1969 4 Sheets-Sheet 2 Dec. 12, 1972 J WEATHERMANET AL 3,705,970

APPARATUS FOR WELDING A CONDUCTOR THROUGH COLD FLOWABLE INSULATIONOriginal Filed July 28, 1969 4 Sheets-Sheet 5 Dec. 12, 1972 WEATHERMANET AL 3,705,970

APPARATUS FOR WELDING A CONDUCTOR THROUGH COLD FLOWABLE INSULATIONOriginal Filed July 28, 1969 4 Sheets-Sheet 4 I i l I a t t 3 g 4 TSW/TCA 92 //3 3 L l sw/ra/ 7a United States Patent US. Cl. 219--86 12Claims ABSTRACT OF THE DISCLOSURE A conductor, surrounded with coldflowable "insulation, is welded to a terminal pin, e.g., by engaging theconductor and the pin between a pair of unheated welding electrodes. Theelectrodes are urged together with a force which reaches a value greaterthan a desired welding force value and is suflicient to cause theinsulation to undergo cold flow out of a path between the electrodes viathe conductor and the pin, thereby to render the path conductive. Theforce applied to the electrodes is then decreased to the desired weldingforce value and a pulse of welding current is passed alongthe conductivepath when the desired force value is achieved. The current pulse isapplied as the force applied to the electrodes decreases and iscompleted, and the molten metal formed by the current pulse issolidified, before such force is completely removed. Th'e conductor issupplied through a passage formed in one of the electrodes.

This is a division of application Ser. No. 857,258, filed July 28, 1969,now Patent No. 3,627,970;

FIELD OF THE INVENTION This invention pertains to an apparatus forWelding conductors and the like to terminal pins or circuit componentsin the fabrication of electrical networks and electronic circuits.

BACKGROUND OF THE INVENTION Review of the prior art Resistance weldinghas been used for some time in the manufacture of precision electronicdevices to conductively connect and mechanically secure conductors toterminal pins on printed circuit boards, to terminal pins of electricalconnectors, and to electronic circuit components, for example.

Most prior welding techniques and machines were developed before therecent emphasis upon compactness and small size of electrical componentsand circuits, and at a time when labor costs were lower. With the adventof smaller circuit assemblies and components, conductor sizes havedecreased to such a point that it is critical that the connection of aconductor to a terminal pin be made effectively upon the first attempt;conductors, particularly the conductors extended from semiconductorintegrated circuit units, are now so small in diameter that if aWeldment of a conductor to a terminal pin is not initially completedproperly, the conductor may be severed so that the weldment cannot beremade. Also, even if the conductor is not severed in making a defectiveweldment, labor costs are now at such level that location and repair ofdefective weldments is often too expensive to be commercially feasible.These factors make it highly desirable that the connection weldment bemade efiiciently, effectively and reliably upon the first attempt. Oncethe conditions necessary to the making of reliable welds have beenestablished, repeatably effective welds can be produced only PatentedDec. 12, 1972 by the use of highly skilled personnel, or by the use ofequipment which assures that the desired conditions are present duringthe welding process. Prior welding techniques and equipment do notadequately produce repeatable welds when applied to current circuitarrangements and production requirements.

United States Patent 3,252,203 describes a method for connecting, bywelding, a conductor surrounded by a cold flowable insulating materialto a circuit board terminal pin. This method requries that both weldingelectrodes be located adjacent the same side of the circuit board. Sucharrangement is subject to the problem that the disposition of thecomponents of the circuit defined on the board, or of other conductorsand terminal pins, may restrict access to the conductor and terminal pinof interest or may be damaged as the board is manipulated to place theconductor and terminal pin of interest in the proper relation to theelectrodes. As a corollary, this arrangement places practical limitsupon the proximity which one terminal pin may have to other terminalpins. Also, the structural arrangement required to practice the methodof this patent is such as to restrict ready visual observation of theconductor and terminal pin being connected, and to require precisepositioning of the conductor and terminal pin relative to the speciallyconfigured electrodes. As a result, the method described in Patent3,252,203 is not well suited to use by relatively unskilled personnel inmaking many welds of highly repeatable nature and high quality in ashort time.

United States Patent 2,977,672 also describes pressure induced cold flowof insulation out of the way between a pair of conductors prior to themaking of a weld in the area of physical contact between the conductors.The structure described in this patent is arranged so that the problemsof restriction of field of view are not present as in the structurecontemplated by Patent 3,252,203. Patent 2,977,672, however, describesstructure in which the forces applied to the conductors during thewelding process are subject to control completely by a human operator.Thus, welds produced by use of such structure are not of repeatably highquality as required by present commercial and military acceptabilitycriteria.

Several other welding methods are also known. For example, Patent 3,155,809 describes a welding method and device in which heated weldingelectrodes are forcefully engaged with the elements to be welded tosoften and displace heat softenable insulation from the path between theelectrodes via such elements. Where such apparatus is used with fineconductors having thin insulator coatings, undesired contact of theelectrodes with the conductor may lead to removal of insulation introublesome locations and result in rejection of the finished product.

Other problems of these and other welding methods and devices arediscussed below.

SUMMARY OF THE INVENTION The present invention provides novel method andap paratus for effectively welding through insulation to connect aninsulated conductor to another conductive element such as anotherconductor or a terminal pin on a circuit board or the like. The methodinvolves practices and procedures which are contrary to previously knownrecommended welding practice and procedure to produce Welds of higherbasic strength than was previously possible. Also, such departures fromrecommended practice and procedure make it possible to use combinationsof electrode and conductor materials which were not previously workable.The apparatus produces highly repeatable welding conditions andeliminate human interference and discretion from the welding process. Asa result, the apparatus is usable by relatively unskilled personnel toprovide, in precision electronic components and circuits, welds of 3high strength and quality rapidly andwith a high degree ofrepeatability.

The present invention makes it possible to provide direct point-to-pointconnections in printed circuit configurations and in wiring arrays inwhich wire-wrap techniques were previously used. In printed circuittechnology, point-to-point connection is often impossible because ofintervening conductors the presence of which require that a givenconductor go around the intervening conductor. In wire-wrap techniques,the mass of wires already in place on the circuit board may make itimpossible to go directly from one terminal to another. In the practiceof the present method, it is possible to provide straight-lineconductive paths to an extent not heretofore practicable.

Briefly stated, this invention provides a pair ofsubstantiallyalignedwelding electrodes coupled in a series circuit, viaa control switch, to a suitable source of welding current. Theelectrodes are relatively movable along their axis of alignment. Powermeans are connected to the electrodes for biasing theelectrodes towardeach other with increasing and then decreasing force, which forceincreases from and decreases to zero magnitude. The maximum forceapplicable to the electrodes by the power means substantially exceeds apredetermined welding force value associated with the production of aweldment of interest. The control switch is coupled to the electrodes tocomplete the circuit when the force applied to the electrodes equals oris less than the predetermined welding force value. Means are providedto disable circuit completing operation of the control switch duringoperation of the power means to increase the force applied to thewelding electrodes.

TERMINOLOGY In the following description of the invention, as well as inthe appended claims, the terms and phrases set forth below shall beunderstood to have the meanings set forth unless the context whereinsuch terms and phrases are used clearly indicates otherwise. Thesedefinitions are set forth at this point so that the followingdescription and the language of the claims may be simplified.

(1) The work consists of the two items which are to be welded together.In the examples illustrated and described hereinafter, the work consistsof an insulated conductor and a terminal pin mounted to a circuit board.The circuit board may be a conventional organic resin impregnated paper,or equivalent, circuit board, or it may be a ceramic circuit boardhaving through terminal pins fired as by sintering into the ceramicmaterial. The insulated conductor may be a round wire surrounded by asheath of insulating material, or it may be a fiat metal strip embeddedin or carried on a sheet of insulating material. Within the meaning ofthe work as used herein, the terminal pin may be replaced by aninsulated conductor or an uninsulated conductor, for example.

(2) A weld schedule is a description, usually in graphical form, of therelation between applied force and welding power (normally expressed inwatt-seconds) necessary to' produce an acceptable weld between twospecified conductive items by the use of a specified welding machine ortype of welding machine. The weld schedule is presented with respect towelding electrodes of specified shape and composition. A weld scheduleis normally obtained by making several test welds upon the desired workand by analyzing the welds for structural and metallurgical integrity.Those welds found to be acceptable are used to define, on a graph inwhich applied force is plotted against welding power, a zone withinwhich lie the combinations of applied power and force which may be usedto produce acceptable welds.

(3) 'Predeterrnined welding force value is the applied force value whichcorresponds to or closely approximates the optimum applied force withinthe zone of acceptability defined by the weld schedule pertinent to thework and the we d g ppa atus of interest.

DESCRIPTION OF THE DRAWINGS I The above-mentioned and other features ofthe invention are more fully set forth in the following detaileddescription of a presently preferred embodiment of the invention, whichdescription is presented with reference to the accompanying drawings,wherein:

FIG. 1 is a cross-sectional elevation view of a resistance welding headwhich constitutes a portion of a welding machine according to thisinvention;

FIG. 2 is a cross-sectional plan view taken along line 22 of FIG. 1;

FIG. 3 is an illustration of the drive means provided to automaticallyoperate the welding head shown in FIGS. 1 and 2;

FIG. 4 is a cross-sectional view taken along'line 4-4 in FIG. 3; a

FIG. 5 is an enlarged cross-sectional elevation view of the weldingelectrodes, an insulated conductor, and a circuit board intherelation'of such elements during the making of a weld pursuant to themethod of this invention; I

FIG. 6 is an enlarged cross-sectional elevation view of a portion of theillustration of FIG. 5;

FIG. 7 is a fragmentary cross-sectional elevation view of a portion ofthe welding machine in another stage of its operation;

FIG. -8 is a perspective view of a welding machine according to thisinvention; 7

FIG. 9 is a fragmentary'plan view of a portion of a circuit board towhich conductors have been secured by the present welding method;

FIG. 10 is a graphic'illustration of the relation between electrodeforce and time during operation of the machine shown in FIG. 8 toperform the present method; and

FIG. 11 is an enlargement of a portion of the graphic representationshown in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 8 shows a resistancewelding machine 20 which is a presently preferred embodiment ofapparatus for practicing the improved welding method taught by thisinvention. The welding machine includes a box-like, generally horizontalbase 21, and a housing 22 for a resistance welding head 23. Housing 22extends upwardly from'the rear portion of base 21 which also mounts ahorizontal work support table 24 having a central aperture 25. The basealso incorporates an ON/ OFF switch 26 for the welding machine, and acycle switch 27.

The welding machine, as shown in FIG. 8, is used in combination with aresistance welding power supply 30, the output of which is connected tothe input terminals (not shown) of welding head 23 by suitable cables31. The power supply includes its own ON/ OFF switch 32 and suchadditional controls 3-3 as are necessary. The power supply as such formsno part of this invention and may be any suitable commercially availableresistance welding power supply. It has been found that a Model 1-049-03power supply, available from Unitek Corporation, Monrovia, California,or equivalent, may be used to good advantage with the welding machineshown in FIG. 8 and described in greater detail below.

As shown in FIGS. 1 and 2, welding head 23 includes a base 35 which issecurely attached to the base of welding machine 20 within housing 22 tothe rear of work support table 24. The welding head also includes acarrier slide 36 which is mounted to base 35 \by anti-friction meanssuch as ball bearings 37 for only vertical motionrelative to the base.The resistance welding head further includes a drive slide 38 which issimilarly mounted for movement only linearly in a vertical directionrelative to base 35; drive slide 38 is supported for such movement bysuitable anti-friction means such as ball bearings 39. Slides 36 and 38,therefore, may move freely in linear reciprocal motion relative to base35 and relative to a lower welding electrode 40 supported in anelectrode holder horn 4-2 which is mounted to base 35. Electrode 40 isengaged in the unsupported end of horn 42 which extends in cantileverfashion forwardly from an electrode arm 41 which is rigidly mounted tobase 35- via electrical insulation 83 under work support table 24 in thebase of Welding machine 20. The upper portion of electrode 40 extendsthrough aperture 25 in work support table 24 and has a frusto-conicalconfiguration.

A drive slide rod 44 is connected to drive slide 38 and extendsdownwardly through base 35 to power means 8-5 (see FIGS. 4 and and thefollowing description relative thereto) which is operable to provide adisplacing force for moving the carrier and drive slides reciprocallyrelative to base 35 and electrode 40*. A relative motion force adjustingknob 45 is connected to a relative motion force adjusting shaft 46extending through base 35 from the upper extent of the drive slide.

As shown in FIG. 1, carrier slide 36 defines a tongue 48 which projectsrearwardly through a central aperture 49 (FIG. 2) in the drive slide.The upper surface of tongue 48, within aperture 49, defines a springseat 50 for receiving the lower end of a compression spring 51, theother end of which is engaged with a nut 52 which is threadably engagedwith force adjusting shaft 46 and which cooperates within aperture 49 soas to be constrained from angular movement relative to drive slide 38.Spring 51 is disposed circumferentially of the lower end of shaft 46. Aretainer ring 62 is engaged circumferentially of force adjusting shaft46 just below the upper extent aperture 49 so as to be secure frommovement axially of the shaft. A washer 63- is engaged between theretainer ring and the drive slide to prevent spring 51 from moving shaft46 upwardly relative to the drive slide. Knob 45-, therefore, isrotatable to vary the compressive bias programmed into spring 51 therebyto adjust the force which must be applied upwardly to the carrier slideto drive the carrier slide upwardly relative to drive slide 38 Carrierslide tongue 48 has a first threaded bore 53 through it rearwardly,i.e., to the left in FIG. 1, of spring '51. A second threaded bore 54extends through the rear end of the tongue. The drive slide defines aswitch mounting lug 55 immediately below drive slide aperture 49, and astop lug 56 adjacent the opposite extremity of aperture 49. The stoplug, as shown in FIG. '1, cooperates with base 35 to limit the upwardmovement of the drive slide relative to the base. The carrier slidetravels with the drive slide relative to base 35 except when the carrierslide is driven relative to the drive slide against the bias of spring51. It is apparent that lug 56 also defines the normal or at-restposition of a second electrode 58 which is carried in the unsupportedend of a holder horn 59. Horn 59 extends forwardly from an electrodesupport arm 60 defined by the carrier slide forwardly of tongue 48.Electrode holder horns 42 and 59 are of such length that the electrodes40 and 58 are essentially coaxially aligned with each other. As shown inFIG. 1, electrode support arm 60 projects from the carrier slide throughan aperture 61 formed in the front face of welding machine housing 22above work support table 24.

A horizontal bore 65 is formed through the carrier slide centrally belowspring seat 50. A pin 66, having a diameter substantially smaller thanthe diameter of bore 65, extends through the bore and has its oppositeends secured to drive slide 38.

A grounding strap 68 has one end secured to carrier slide tongue 48 bymeans of a nut 69 and a bolt 70 which cooperate with bore 54. The otherend of the grounding strap is secured to welding head base 35 by a nut71 and a bolt 72-.

Drive slide rod 44 passes from power means 85 to the drive slide throughan aperture 73 formed in base 35 and which is fitted with a retainingwasher 74. A compression spring 75 is disposed circumferentially of thedrive slide rod between the drive slide and washer 74 to bias the driveslide upwardly relative to the welding head base. Spring 75 urges thedrive slide into its upper limit of motion relative to base 35, at whichupper limit stop lug 56 is engaged with the base as shown in FIG. 1.

Since spring 51 acts to urge the carrier slide downwardly relative tothe drive slide, pin 66 is normally engaged with the upper extremity ofbore 65, as shown in FIG. 1. In use of the welding head, however, aswill be made clear from the following description, the drive slide isdriven downwardly relative to base 35 by operation of power means 85 towhich drive slide rod 44 is connected. If, during movement of the driveslide downwardly relative to the base, electrode 58 should engage thework to be welded together, the carrier slide will encounter resistanceto movement with the drive slide downwardly relative to base 35. Thisresistance to continued downward movement of the carrier slide worksagainst the bias imposed upon the carrier slide by spring 51. Dependingupon the force applied to upper electrode 58 by the work, and upon thebias applied by spring 51 to the carrier slide, the carrier slide willultimately become stationary relative to base 3-5 as downward movementof the drive slide continues. The bias programmed into spring 51,therefore, determines the amount of force which must be applied toelectrode 58 to cause the carrier slide to cease moving downwardly withthe drive slide during operation of the power means. Spring 51 alsodetermines the rate at which force is further applied to the work heldbetween the adjacent ends of electrodes 40 and 58 during furtherdownward movement of the drive slide.

The difference in the diameter between bore 65 and pin 66 determines themaximum amount of movement of the carrier slide upwardly relative to thedrive slide during operation of the power means. Since upward movementof the carrier slide relative to the drive slide produces deflection ofspring 51, it is apparent that the maximum force applicable by spring 51to electrode 58 relative to electrode 40 is determined by the stiffnessof spring 5 1 and its initial state of deflection in the illustratedat-rest condition of the welding head. That is, with respect to FIG. 10(the same being a graphical representation of the relationship of theforce applied by electrodes 40 and 58 to work disposed between themduring the time required for operation of the power means shown in FIGS.3 and 4 through one cycle), the stiffness of spring 51 determines theslope of curve 77, and the difference in diameter between bore 65 andpin 66 is correlatable to the maximum value of the peak of the curve fora given spring 51 and the initial bias state of such spring. Inpractice, however, it is preferred that pin 66 not engage the lowerextremity of bore 65.

A normally closed limit switch 78 is mounted on switch mounting block 55of drive slide 38 and has a pair of leads 79 extending from the switch.Switch 78 has a switch arm 80 which cooperates with a set screw 81engaged in threaded bore 53- of carrier slide tongue 48. Switch 78,therefore, is sensitive to movement of carrier slide 36 upwardlyrelative to drive slide 38. Since upward movement of the carrier sliderelative to the drive slide produces deflection of spring 51, and thedeflection of spring 5-1 determines the force which is applied byelectrode 58 to work held between electrode 58 and electrode 40, itfollows that switch 78 is sensitive to the force applied by theelectrodes to work engaged between them. The contacts of switch 78 areclosed and remain closed until the force exerted by spring 51 uponcarrier slide 16 exceeds a value which is predetermined by the settingof set screw 81 relative to the carrier slide, and by the stiffness andinitial bias of spring 51. During downward movement of drive slide rod44 in'response to operation of the power means, the contacts of switch78 remain closed until the pretermined force value has been reached, atwhich time the contacts open and remain open until the predeterminedforce value is again reached on the upstroke of rod 44. It will beapparent from the foregoing description that downward movement of rod 44relative to base 35 produces increasing force between the electrodes,and upward movement of the rod relative to the welding head base corresponds to decreasing force between the electrodes.

Switch 78 is disposed in series relation between elec- 'trodes 58 and40' and the output of resistance welding power supply 30. An additionalswitch 92, forming a part of power means 85, is also placed in serieswith switch 78 between the electrodes and the power supply to disableeffective coupling of the power supply to the electrodes may be used toadvantage in welding machine 20. At the present time, it is preferredthat a Model 2032.-03 resistance welding head available from UnitekCorporation, Monrovia, California he used as a component of the weldingmachine. In this regard, it is important to note that a Unitek Model2-032-03 resistance welding head incorporates a normally open switch inthe place of normally closed switch 78 described above. Thismodification is germane to the present invention, and the significanceof such modification will be apparent from the following descrpition.

Lower electrode arm 41 is insulated from base 35 by insulation 83.Welding current is supplied to electrode arm 41 via a suitable conductor(not shown), the energization of which is controlled by switch 78 andswitch 92. Similarly, the carrier slide, which is placed out ofconductive contact with housing 22 of welding machine 20, is connectedby a suitable conductor (not shown) to the power supply via cable 31.

FIGS. 3 and 4 illustrate power means 85 to which welding head 23 iscoupled via drive slide rod 44. The power means includes a shaft 86which is supported for rotation in stationary bearings 87. A control cam88 and two switch operating cams 89 and 90 are secured to shaft 86. Apair of microswitches 91 and 92 cooperate with cams 89 and 90,respectively. Shaft 86 also carries a gear 93 which cooperates with apinion gear 94 carried by the output shaft 95 of an electromagneticclutch 96. The input shaft 97 to the clutch is the output shaft of anelectric motor 98. Motor 98 and clutch 96 are coupled in series across apair of power means energization terminals 99 via a foot operated switchI100. Closure of switch 100 energizes both clutch 96, to couple shafts97 and 95, and motor 98. Switch 91 is coupled in parallel with switch100 and is a normally open switch which, because of the profile of cam'89, is closed promptly upon the first increment of rotation of shaft 86and remains closed until just before shaft '86 has completed one fullrotation from its base position. Each of switches 91 and 92 includes aresiliently biased switch arm 101 which extends in cantilever fashionpast a switch actuating plunger 102 and carries a cam follower roller103 at its unsupported end.

As shown best in FIG. 4, a cam follower roller -5 is rotatably mountedto a bifurcated end of a bell crank 106 for cooperation with theperiphery of control cam 88. The bell crank is pivoted intermediate itsends to a stationary fulcrum 107. The end of the bell crank oppositefrom cam follower roller 105 is connected to the lower end of driveslide rod 44 of resistance welding head 23. Cam 88 has a rest portion109 in its periphery along which the radius of the cam from shaft '86 isleast. Cam rest portion 109 corresponds to the at-rest position of thestruc ure of the wel g head a d, when fol ower roller is engaged withcam rest portion 109, the structure of the welding head is in the stateshown in FIG. 1. The cam also has a control lobe 110 extending over themajor portion of the remainder of the periphery of the cam, thevariation of the radius of the cam from the smallest diameter thereofbeing essentially sinusoidal in nature over the extent of the camcontrol lobe. Thus, the profile of cam 88 along control lobe 110determines the character of curve 77 in FIG. 10. The beginning of thecontrol lobe corresponds to the origin of the graphical representationin FIG. 10, and the end of the lobe corresponds to the point at whichcurve 77 returns to the F=0 ordinate in such graphical representation.

In FIG. 10, curve 111 describes the condition of motor control switch 91throughout one full cycle of the opertion of the welding machine; onefull cycle of the welding machine corresponds to one full rotation ofshaft 86. Switch 92 of power means 85 is an inhibit switch which isconnected in series with switch 78 in welding head 23, and curve 112 ofFIG. 10 describes the operation of this switch during one cycle ofoperation of the welding machine. Curve i113 in FIG. 10 describes theoperation of switch 78 in the welding head. Curve 111 illustrates thatswitch 91 closes momentarily after operation of switch 100 to maintainenergization of motor 98 after ,the operator of welding machine 20removes his foot from switch 100. Curve 112 illustrates that cam 90 andcam 88 are phased relative to each other on shaft 86 'so that switch 92closes at about the time that electrodes 40 and 58 exert maximum forceupon the work engaged between the electrode tips, thereby enablingswitch 78 to trigger the firing of a pulse of welding current from powersupply 30 through the electrodes at the time a predetermined weldingforce value (programmed into the welding head by the adjustment of setscrew 81, and by the character and adjustment of spring 51) existsbetween the electrodes. 7

FIGS. 1 and 2, and particularly FIG. 5, illustrate that upper electrode58 is axially hollow to define a vertical passage 115 through which aninsulated conductor 116 is fed to the gap 117 which exists between theelectrodes from a conductor supply spool 118. As shown in FIGS. 1 and 8,the conductor supply spool is mounted to welding machine housing 22above the location at which upper electrode holder horn 59 projects fromthe welding head.

FIG. 9 illustrates an exemplary electrical component 120 in thefabrication of which the method and apparatus of this invention are usedto advantage. Component 120 may be a printed circuit board 121 throughwhich a plurality of conductive terminal pins 122 are fitted at desiredlocations on the circuit board. The surface of the board which isvisible in FIG. 9 may be regarded as the reverse surface 123 of theboard, the obverse surface of the board carrying a pattern of conductivetraces and selected circuit components such as resistors, capacitors,and the like. It may be desired to interconnect various terminal pins byconductors 124, 125, 126, 127, and 128- in a predetermined manner acrossthe reverse side of the circuit board. The method and apparatus of thisinvention allows these conductors to be welded to terminal pins 122rapidly and effectively without the need of stripping insulation fromthe conductor stock material. (Conductors 124-428 are defined by piecesof conductor 1.16 which is sometimes referred to as the stock conductoror the conductor stock material.) In effect, conductors 124-128 arestitched to the terminal pins in the desired pattern by a procedurewhich is both efficient and reliable.

As shown in FIG. 5, terminal pins 122 are fitted in correspondingapertures 129 defined at desired locations in circuit board 121. Eachterminal pin has an upper surface 130 disposed adjacent the circuitboard reverse surface, and a lower end surface 131. FIG. 5 illustratesthat the upper ends of the terminal pins define an enlarged head 132 butthis feature of the terminal pins is not required for succes fulpractice of the method of this invention.

All that is desired is that the pins be configured so that they aresecure within the desired apertures of the circuit board. Also, and thisis important, it is desired that terminal pins 122 be uniform in lengthbetween opposite end surfaces 130 and 131. It is not necessary thatterminal pin upper end surfaces 130, for example, be uniformly spacedfrom board surface 123. For emphasis, all that is necessary is that thepins be of uniform and known length between their end surfaces; thereason for the significance of this characteristic of the terminal pinswill be explained in greater detail below.

In FIG. 6, which is an enlargement of a portion of the illustration ofFIG. 5, conductor 116 is fed down upper electrode passage 115 fromconductor supply spool 118 and has a core defined by a length ofconductive wire 133 of suitable diameter. The wire is surrounded alongthe length of the conductor by a sheath of electrical insulatingmaterial 134.

In order that conductors 124-128 may be secured to terminal pins 122 inthe arrangement shown in FIG. 9 Without stripping the insulation fromthe conductor stock material, it is important to the practice of themethod of this invention that insulating material 134 be cold flowableunder applied pressure. That is, insulator material 134 should have theproperty that when pressure is applied to the material, the materialdeforms and tendsto flow laterally from the point of greatest pressure.The specific cold fiowable insulating material utilized in a conductorcontemplated by this invention is not critical. Suitable insulatingmaterials may be provided by fluoroethylene propylene,tetrafluoroethylene, or nylon. As a practical matter, however, theinsulating material should not be readily cold fiowable under relativelow values of pressure because, if such is the case,- the insulation mayopen in the completed component in response to localized pressures whichmay be experienced by the conductor in use of the component.

As shown in FIG. 5, the upper end of lower electrode 40 defines a flatsurface 136 which is disposed normal to the elongate extent of theelectrode; i.e., surface 136 is disposed perpendicular to the axis ofalignment of the electrodes, such axis being the same line along whichelectrode 58 is movable toward and away from stationary electrode 40.The lower end of electrode 58 is smoothly curved as at 137circumferentially of the opening defined at the lower end of conductorpassage 115. Preferably, the outer diameter of electrode 58 at itsextreme lower end is less than the diameter of the electrode along themajor portion of its length. The reduction in the diameter of theelectrode at its lower end provides for concentration of the forcedeveloped between the electrodes by operation of power means 85 to alocalized small area of conductor 116, and also makes it easier for anoperator of machine 20 to clearly observe the relationship of conductor116 and a desired terminal pin 122 during the formation of a weldbetween wire 133 and terminal pin 122, as shown in FIG. 6.

Before proceeding further with an explanation of the operation ofwelding machine 20, the novel welding method provided by this inventionis described briefly. As shown in the drawings, the welding method ispracticed to weld a conductor 133 surrounded by a cold fiowableinsulating material 134 to a circuit board terminal pin 122. First, theunprepared insulated conductor and the conductive element to which theconductor is to be secured are engaged with each other under theinfluence of a force which is (l) suflicient to produce cold flow of theinsulating material from between the conductor and the conductiveelement, and (2) greater than the predetermined welding force valuedefined by the weld schedule for the work and electrodes underconsideration. Next, the force with which the conductor and the elementare engaged is reduced to the predetermined welding force 'value.Preferably, the reduction in the force applied to the work is reducedfrom the maximum value of the force to the selected welding force valuesufiiciently rapidly that the cold fiowable insulating materialsurrounding the conductor does not have an opportunity to flow back intothe path between the electrodes through the work, which path wasrendered conductive by the practice of the first step of the method.Once the force applied to the work is reduced to the selected weldingforce value, a pulse of welding current is passed through the electrodesto produce the desired weld. The power of the pulse of welding currentwill be in accord with the Weld schedule applicable to the work underconsideration. The welding current pulse may be, and preferably is,passed through the work at a time when the force applied to the work bythe electrodes is decreasing; this aspect of the present methodconstitutes a radical departure from previously recommended weldingpractice and procedure.

With the foregoing summary of the method of this invention in mind, theoperation of welding machine 20 to produce the product shown in FIG. 9should be apparent. Assume that the first stage in the production ofcomponent 120 is to connect conductor 128 to terminal pins 122', 122"and 122". Circuit board 121 is disposed on work support surface 25 ofthe welding machine with surface 123 disposed upward. Board 121 ispositioned so that terminal pin 122 is located over hole 25 and has itslower end 131 engaged with the fiat surface 136 defined at the upper endof lower electrode 40. At the time the circuit board is so disposed onthe work support surface, the upper electrode is retracted from thelower electrode and the structure of welding head 23 is in the stateshown in FIG. 1. When the operator of machine 20 determines visuallythat terminal pin 122 is suitably disposed on electrode 40, foot switchis operated to activate power means 85. It will be seen that operationof switch 100 causes motor 98 to become effective via gears 93 and 94 torotate shaft 86 and cam 88. Cam 88 cooperates with cam follower to firstdraw welding head drive slide rod 44 downwardly relative to the base ofthe welding head, and then to allow the rod to be moved upwardly underthe bias of spring 75 as roller 105 approaches the end of cam controllobe 110. The contour of cam 88 is defined so that when roller 105 is atthe beginning of lobe 110, conductor 116 is moved into contact withterminal pin 122 but the force applied by the electrodes to the terminalpin and the conductor is essentially zero so that insulation material134 surrounding conductor wire 133 is undeformed. The effective throw ofcam 88, i.e., the difference in the radius of the cam from the beginningof lobe to the point of maximum radius of the lobe, is defined withreference to the length of terminal pin 122 between surfaces and 131 andthe diameter of conductor wire 133. Specifically, the throw of cam 88through cam control lobe 110 is slightly greater than the differencebetween the length of terminal pin 122 and the diameter of conductor116, on the one hand, and the length of pin 122 and the diameter ofconductor wire 133, on the other hand. Also, set screw 81 in weld head23 is adjusted so as to cause operation of switch 78 when the pressureapplied to the work by electrodes 40 and 58 corresponds to thepredetermined welding force value indicated by the applicable weldschedule. Adjusting knob 45 is operated to adjust the bias of spring 51so that the maximum force applied to the work by the electrodes duringrotation of shaft 86 is substantially greater than the predeterminedwelding force value, and so that pin 66 does not bottom out at the lowerextent of bore 65. Thus, during that portion of the rotation of shaft 86within which cam follower roller 105 traverses cam control lobe 110,drive slide rod 44 is first moved downwardly and then upwardly relativeto the base of the welding head to first increase and then decrease theforce applied by the electrodes to conductor 116 and terminal pin 122.

The shape of the curve which is produced when electrode force is plottedgraphically against time for a full rotation of shaft 86, i.e., curve 77in FIG. 10, is essen- 11 tially identical to the curve (not shown) whichdescribes the rate of rise and fall of the cam along cam control lobe110 relative to the base diameter of the cam, i.e., to the diameter ofthe cam at the beginning and end of the lobe.

The maximum force applicable by the electrodes to the work is selectedso that such force is adequate to produce cold flow of insulatingmaterial 134 from between the lower end of electrode 58 and conductorwire 133, and from between the conductor wire and terminal pin 122,thereby to render conductive the path from the upper'electrode to thelower electrode through the conductor wire and the terminal pin.

At the time cam 88 begins to rotate following closure of foot switch100, the contacts of power supply inhibit switch 92 are opened by reasonof the profile of cam 90 and do not close until about the time camfollower roller 105 reaches the point of maximum radius of cam 88. Sinceswitch 92 is coupled in series with power supply control switch 78,switch 78 is rendered inoperative to trigger a welding pulse to theelectrodes during the initial stages of downward travel of the upperelectrode. Because the contacts of switch 92 are not closed until theforce applied to the work by electrodes 40 and 58 reaches a valueexceeding the predetermined welding force value indicated by theposition of set screw 81, no welding pulse can be applied to theelectrodes until the force applied to the work by the electrodes fallsto the predetermined welding force value on the upstroke of drive sliderod 44.

Because force is applied to conductor 116 and terminal pin 122 on theupstroke of rod 44 up to and through the time of passage of the pulse ofwelding current through the electrodes, the cold flowable insulatingmaterial surrounding conductor wire 133 cannot reenter the spaceoriginally occupied by this material, and the path between theelectrodes through the work remains conductive. Thus, as the forceapplied to the conductor and the terminal pin falls to the predeterminedwelding force value, the path 'between the electrodes is stillconductive. At the time the applied force reaches the predeterminedwelding force value to which switch 78 is sensitive, this switch closes(switch 92 having previously been closed) to trigger power supply 30to-fire a pulse of welding current having the intensity and durationindicated by the appropriate weld schedule. The profile of cam 88 isconfigured so that the pressure applied to the work following firing ofthe welding pulse is not completely relaxed from the work until currenthas ceased to flow through the electrodes and the puddle 139 (FIG. 6) ofmolten metal produced between conductor wire 133 and terminal pin 122 bysuch current has had an opportunity to solidify.

It is apparent, therefore, that Welding machine 20 1s effective toproduce a weld under precisely the conditions indicated by the weldschedule applicable to the work of interest. These conditions areproduced automatically; any action taken by the operator of machine 20following closure of switch 100 has no effect upon the operation of themachine or the character of the weld produced by the machine. Thus, allthat is necessary on the part of the operator is to properly positionthe circuit board in the gap between electrodes 40 and 58, and tooperate foot switch 100.

Once the welding machine has operated to make the weld of conductor 116to terminal pin 122', the machine operator then moves the circuit boardon work support table 24 to position the terminal 122" on electrode 40.Such movement of circuit board 121 laterally of hole 25 causes anappropriate amount of conductor 116 to be drawn from supply spool 118and passed through hollow electrode 58. Once terminal pin 122 has beenproperly positioned on the upper end of the lower electrode,

switch 100 is again closed by the operator and the machine functionsautomatically to make the second desired weld, which Weld is performedunder the same conditions as the first weld and is of strength andquality equal to the first weld. The circuit board is then adjusted toplace terminal pin 122" in position between the electrodes and thedesired weld is made by the machine. In this way, conductor stock 116is, in efiect, stitched by welding to the desired terminal pins ofcircuit board 121 to define conductor 128, illustrated in FIG. 9. Afterthe last weld associated with conductor 128 has been made, the operatormerely moves the circuit boards slightly from the electrodes and cutsconductor 116 adjacent the last terminal pin associated with conductor128. The operator is then in a position to connect conductors 124-127 totheir respective terminal pins merely by repeating the process describedabove an appropriate number of times. Each weld made to the terminalpins of circuit board 21 is of uniform strength and metallurgicalquality.

It has been determined that the shape of curve 77 (see FIG. 10) isimportant to the successful practice of this invention. It has beendetermined, relative to operability of the invention rather than thepracticality or efiiciency thereof, that the rise and fall portions ofthis curve cannot have too small a slope. Similarly, it has beenascertained that the rise and fall portions of the curve, particularlythe fall portion, can have too steep a slope. If the slope of curve 77in its rise portion, i.e., the porticn thereof between the intersectionof the F and T coordinates in FIG. 10 and the peak of curve 77, is toost: ep, insulating material 134 may not have adequate time to respond tothe force applied to it and may not flow outwardly from between thelower end of electrode 58 and conductor wire 133 or from between thewire and terminal pin 122, before the pulse of welding current is firedon the upstroke of drive slide rod 44. If the slope of curve 77 alongthe rise thereof is made too gentle, as by stretching the curve alongthe time coordinate by reducing the rate of rotation of shaft 86, forexample, all that is lost is productive machine and operator time sincethe number of welds which an operator can make in a given period of timeis unnecessarily reduced.

For a given type and thickness of cold flowable ins lating material,experience will rapidly define the upper workable limit of the slope ofcurve 77 in its rise portion.

It has been found that best welds are produced by operation of weldingmachine 20 when the bias of spring 51 is adjusted so that the maximumforce applied to the work by electrodes 40 and 58 is suflicienttoburnish the metal of conductor wire 133 at the interface of the wirewith terminal pin 122. Also, the maximum applied force should not be sohigh as to appreciably pinch and flatten wire 133 against pin 122.Obviously, an applied force sufficient to cause electrode 58 to severwire 133 is to be avoided.

Also, if the slope of curve 77 in its fall portion, i.e., the portion ofthe curve having negative slope, is too gentle (has exceptionally lownegative slope), all that is lost is time. However, if the slope ofcurve 77 is too great, in a negative sense, at the time the pulse ofwelding current is applied to the electrodes, the welding pulse may notbe terminated at the time the loading force of the electrodes upon thework is removed, thereby enabling the conductor wire to separate fromthe terminal pin before the puddle of molten metal necessary to producethe weld has solidified. Therefore, as a minimum, the time between theinstant of firing of the pulse of welding current and the instant atwhich the force applied by the electrodes to work is removed must beatleast as great as the duration of the welding current pulse and the timerequired for the weld puddle to solidify.

FIG. 7 illustrates two accessories which may be used to advantage withthe structure of welding machine 20. To cause conductor 116 to lieclosely against the upper surface of circuit board 121 during thestitching of conductor 116 to selected ones of terminal pins 122, anannular guide ring 142 is supported above work support table 24concentric to upper electrode 58. The g ide ring is located between thelower end of electrode 58, in its upper (retracted) position, and theupper surface of the circuit board. The inner diameter of ring 142 issufficiently large that electrode 58 can reciprocate freely toward andaway from electrode 40 without interference with the guide ring. Thelower surface of the guide ring is smoothly curved convex downwardly at143 to enable conductor 116 to be draw smoothly past the ring withoutcausing the ring to tear or otherwise damage the insulation of theconductor. The lower extremity of the ring is disposed sufficientlyabove table 24 that circuit board 121 may be passed easily under thering into engagement with the upper end of lower electrode 40. Guidering 142 is supported above table 24 by a support arm 144 cantileveredfrom a suitable post or other mounting bracket located either on thetable remote from electrode aperture 25, or on the base of the weldingmachine adjacent the table.

To facilitate cutting conductor 116 at appropriate points during thefabrication of circuit component 120, for example, a pair of cuttingblades 145 having cooperating cutting edges 146 are disposed adjacentthe gap between electrodes 40 and 58. Each cutting blade is connected toan operating rod 147 which in turn is connected to its own reciprocatingmechanism. The cutting blade reciprocating mechanisms preferably aredisposed sufliciently far from the electrodes that operator access tothe gap between the electrodes is not impaired and so that visual accessto the same area is not restricted. The reciprocating mechanisms may besolenoids, for example, operated in tandem by a suitable switch mountedto the structure of welding machine 20. Preferably the lower surfaces ofcutting blades 145 are slidably engaged with the upper surfaces ofconductor guide ring 142. Upon actuation of the reciprocatingmechanisms, the cutter blades move into the position shown in dottedlines in FIG. 7 to sever conductor 116. Preferably the cutting bladesare operated following the making of the last weld of a desired lengthof conductor 116 to a suitable one of terminal pins 122 and before thecircuit board is moved laterally of the electrodes following the makingof such weld.

It is believed that success in the production of repeatedly acceptablewelds by the use of the present method and apparatus is dependent uponsix variables, namely, (1) the duration of the pulse of welding currentapplied to the Work, (2) the force applied to the work at the time theWelding current pulse is applied, (3) the magnitude of the differentialof applied force with respect to time (dF/dt) at the time the weldingpulse current is applied, (4) the sign of dF/dt at the instant of firingof the welding current pulse, (5) the electrode alloy, and (6) thepolarity of the welding current pulse. The matters of the magnitude ofseveral of these variables and the effect of variations in the magnitudeof such variables have been discussed to some extent above, particularlywith reference to welding current pulse duration and the magnitude ofthe applied force at the time the welding current is applied to thework.

The force applied to the work at the time welding current is appliedshould be essentially the optimum welding force as defined by thepertinent weld schedule, i.e., the predetermined welding force value asdefined above. The criteria which should be considered in defining theweld schedule are set forth in available publications of the RWMA(Resistance Welder Manufacturers Association), as are illustrations ofthe indicia of too high or too low a value of welding force and theeffects of too much or too little current in the applied welding currentpulse. Although the present welding method departs radically fromRWMA-recommended practice and procedure, RWMA criteria for evaluatingwelds made by use of the method are generally pertinent. Thus, if theapplied force is too high at the time the welding current pulse istriggered by switch 78, the welding current will be spread out over toolarge an area between wire 133 and pin 122 and an inadequate weld puddlewill result. If the applied force is to 14 low at such time,insufficient pressure will exist to contain the weld puddle and the weldwill explode. In the practice of the present method, the predeterminedwelding force value is approximately of the same magnitude as thewelding force value indicated by the weld schedule for the same work tobe Welded according to a method consistent with existing RWHA practicesand procedures.

The duration of the applied Welding current pulse productive of the bestwelds is normally an item of information given by the pertinent weldschedule. That is, while the weld schedule is plotted in terms of powerexpressed in units of watt-seconds, best practice concerning weldschedules teaches that once the zone of acceptability has been definedfor the work of interest, an optimum pulse duration also is described;once optimum pulse duration has been defined, it is a simple matter todetermine the current intensity necessary to produce the appropriatepower. In general, however, it may be stated that if the pulse durationis too short, erratic conditions will be manifested at the location ofthe desired Weld because current density is too high. On the other hand,if the pulse duration is too long, insufiicient heating of the work mayoccur with resulting lack of penetration in the weld. A pulse durationin the range of from 1.5 to about 10 milliseconds has been found toproduce good results.

The magnitude of dF/dt at the time of firing of the welding pulse hasbeen discussed above in terms of the slope of curve 77 (see FIG. 10). Itis recommended that this differential be such that, during the durationof the weld current pulse and solidification of the weld puddle, theapplied welding force does not decrease from the predetermined weldingforce value by more than about 40 percent. Thus, with reference to FIG.11, if the predetermined welding force (F for the work of interest asdetermined by the pertinent weld schedule is 2 pounds, the appliedwelding force should not decrease to less than about 1.6 pounds (Fduring both the duration At of the welding pulse and the time t to trequired for the weld puddle to become solid.

It has been found that the sign of dF/dt at the time of firing of thewelding pulse is significant to the quality of the weld produced byWelding machine 20. Adherence to RWMA recommendations which teach thatthe applied welding force be constant or increasing during the durationof the welding pulse, results in welds having approximately 70 percentof the strength of the parent materials constituting the work. It hasbeen found, however, that practice of the present method upon thepresently preferred welding machine produces welds having percent ormore of the strength of the parent metals constituting the work.

The polarity of the applied welding pulse may vary depending upon thespecific work and the particular electrode alloys involved in aparticular welding situation. As a general rule, however, it isrecommended that the welding current power supply be connected to thewelding machine so that the upper electrode is negative with respect tothe lower electrode.

RWMA-recommended welding practices and procedures teach that theelectrodes generally should show a thermal conductivity opposite to thematerials constituting the work of interest. It has been found that themethod described above makes it possible to use electrodes having thesame characteristics of thermal conductivity as the materials beingwelded together. It is recommended that the electrodes used in machine20 have the same or higher value of K (thermal conductivity) as the workof interest. While the specific reasons supporting this recommendationare not fully understood, it is believed that the use of electrodeshaving higher values of K than the work enables the electrodes to expandand, in effect to physically follow the work during the continuance ofthe welding pulse and immediately thereafter while the applied weldingforce is decreasing. That is, the use of electrodes having the same orhigher values of K than the work artificially reduces the magnitude ofdF/dt otherwise indicated by the profile of cam 88.

Also, the present method makes it possible to use combinations of workmaterial and electrode alloys which were not heretofore successfullyusable in resistance welding. One of the presently preferred uses of thepresent method and apparatus is to weld PEP-insulated nickel wire tostainless steel terminal pins in circuit boards and the like, asillustrated in FIG. 9. RWMA recommendations, at the time the presentmethod was developed, were very clear on the point that stainless steelelectrodes could not be used successfully with nickel. RWMA literaturevery clearly suggested that a stainless steel electrode and a nickelworkpiece would stick. It has been found that the method described abovemakes it possible to constitute upper electrode 58 of stainless steel.Series 300, and particularly 302 and 303 stainless steels, have beenused to advantage to define electrode 58 in welding machine 20. Further,the electrode and workpiece combinations recommended by the RWMAcontinue to be useful in the practice of method and apparatus accordingto this invention. Thus, in the specific instance mentioned above wherewelding machine is used to weld nickel conductor wire to a stainlesssteel terminal pin, it is preferred that lower electrode be constitutedof RWMA No. 2 electrode material.

While it should be apparent to Workers skilled in the art to which thisinvention pertains that the method and apparatus described above can beused with myriad combinations of workpieces, a presently preferredwelding machine has been used successfully to weld Series 300 stainlesssteel circuit board terminal pins to 28, 30 or 32 gauge work isapproximately 2 pounds and the power of the welding current pulse isbetween 4 and 5 watt-seconds.

It was mentioned above that a Unitek Model 2-032-03 resistance weldinghead is a suitable commercially available welding head useful as acomponent of welding machine 20; this welding head is designed tooperate under conditions where welding force is constant or increasesduring continuance of the welding current pulse. Unitek Corporationrecommends that the optimum inertia of the upper electrode horn be onthe order of 2 ounces. It has been found that best results are obtainedin welding machine 20 when the above-mentioned Unitek welding head ismodified to manifest 3 ounces of upper electrode horn inertia. It isbelieved that this increase in horn inertia makes it possible for theupper electrode to better follow and conform to the work during theactual formation of the desired weld as the applied welding forcedecreases.

A Unitek Model 2032-03 welding head is essentially pressure-sensitivewelding head. In such a welding head, the switch affixed to switchmounting lug is a normally open switch which is coupled directly betweenthe power supply and the electrodes and is effective to trigger thefiring of a welding pulse upon closure. Because of this arrangement ofthe switch in the conventional Unitek resistance welding head, it isclear that these welding heads are designed for use in situations wherewelding force remains constant or increases following closure of theswitch. If the case were otherwise, opening of the switch during thecontinuation of the welding pulse would interrupt the flow of weldingpower to the electrodes. Because a conventional welding head isconstructed to produce firing of the welding pulse during a downstrokeof the upper electrode relative to the work, the desired weld iscompleted before the welding machine operator can manually drive thewelding head drive slide downward to a point corresponding to the peakof curve 77; even if the welding head is operated to a pointcorresponding to the peak of curve 77, such operation is not troublesomebecause the weld has already been completed. For these rea sons, it isnot important when the welding current pulseis triggered in aconventional welding machine where such triggering occurs on thedownstroke of the upper electrode, nor is the maximum force applied tothe work by the electrodes particularly important.

Welding machine 20, on the other hand, is a stroke-sensitive device inwhich displacement rather than applied welding force, is the primaryfactor of concern. Since the throw defined by control cam lobe 110 istranslatable directly into displacement of drive slide 38 relative towelding head base 35, the throw of the control cam lobe must bedetermined with respect to the dimensions of the pertinent work and themaximum force which is to be applied by the upper electrode to the workat the peak of curve 77. The value of the applied force at the peak ofcurve 77 is dependent uponthe deflection of spring 51, such deflectionagain being translatable into displacement. The maximum extent to whichspring 51 is deflected is in turn dependent upon the length of terminalpin 122 and the thickness of conductor wire 133. Too high a maximumvalue of applied welding force can cause conductor 116 to be severed.Thus, the maximum force applied to the work by the welding electrodes inthe practice of the present method becomes a factor of considerableimportance.

Also, in welding machine 20 the weld current pulse is triggered on theupstroke of the upper electrode. It is important, therefore, that thepoint at which the welding current pulse is triggered be controlledwithin relatively narrow limits within the range of displacement of theupper electrode to assure that the upper electrode does not separatefrom the work before the weld current pulse has ended and the weldpuddle has solidified. Maximum applied force and pulse triggering arerelated to the stroke of drive slide rod 44,'to the bias of spring 51,to the diameter of conductor wire 133, and to the length of terminal pin122. The stroke of rod 44 and the bias of spring 51 are constants forany given set-up of machine 20;the knob by which spring 51 is adjustedis within housing 22 and is not readily accessible. Therefore, thediameter of wire 133 and the length of pins 122, as a practical matter,

are the principal variables to control to assure repeatability of thewelds produced by welding machine 20. Since the diameter of conductorwire 133 will not vary significantly in any given use of the machine,the length of terminal pins 122 remains the only real dimensionalvariable the value of which must be controlled to obtain repeatablewelds. It is for this reason that all terminal pins carried by circuitboard 121 should be of uniform length.

The invention has been described above with reference to specificprocedures and examples which have been presented for the purposes ofsetting forth presently preferred embodiments of the invention. Workersskilled in the art to which the invention pertains will most likely,

and quite readily, observe that modifications and alterations to theabove-described procedures and structures can be made to suit specificproblemsfaced by them without departing from the scope and spirit of theinvention. Thereforegoing description should not be regarded as limitingor defining the precise scope of the invention.

What is claimed is: I 1. Apparatus for welding an insulated conductorto' a plurality of interconnected conductive elements, comprising (a) apair of welding electrodes having adjacent work engaging ends, (b) meansmounting the electrodes for guided movement toward and away from eachother, (0) power means for driving the electrodes toward and away fromeach other adequately to engage the con ductor and one of said elementswhen the same are disposed between the electrodes and to apply to suchengaged conductor a maximum force sufficient to part the conductorinsulating material locally between the electrodes to define aconductive path be- 17 tween the electrodes, and then to move apart fromeach other to decrease the applied force to zero magnitude,

(d) sensing means responsive to an interelectrode force of value equalto or less than a predetermined welding force value lying between zeromagnitude and the magnitude of said maximum force for initiating thepassage of a pulse of welding current to the electrodes, and

(e) means effective between commencement of movement of the electrodestoward each other and a point in such movement at which theinterelectrode force exceeds the predetermined welding force value fordisabling effective current initiating operation of the sensing means,

(f) the power means being arranged so that the interelectrode force doesnot decrease, during passage of the current pulse and solidification ofmolten metal produced thereby, by an amount substantially in excess ofabout 40 percent of the predetermined welding force value.

2. Apparatus to claim 1 wherein one of the electrode is essentiallystationary.

3. Apparatus according to claim 1 wherein one of the electrodes definesmeans extending from the work engag ing end thereof for guiding a lengthof the insulated conductor to between the work engaging ends of theelectrodes.

4. Apparatus according to claim 3 wherein the C0111 ductor guiding meansextends along a substantial portion of the length of the one electrode.

5. Apparatus according to claim 4 wherein the con ductor guiding meanscomprises a hole formed substan-Z tially coaxially of the one electrodefrom the Working end thereof to the other end thereof. 1

6. Apparatus according to claim 5 wherein the other electrode isessentially stationary.

7. Apparatus according to claim 1 wherein the elec-j trodes aresubstantially coaxially aligned. i

8. Apparatus according to claim 1 wherein the powerl means is arrangedto apply force in excess of the pre-; determined welding force value fora predetermined period of time to the conductor and one of said conduc-itive elements disposed between the electrodes. I

9. Apparatus according to claim 1 wherein the power means is arranged tocause the applied force to decreas' at a predetermined rate between thepredetermined weld? ing force value and zero magnitude of applied force10. Apparatus according to claim 1 wherein the sensing means comprises afirst switch connected in series relation to the electrodes and themeans for disabling the sensing means comprises a second switchconnected in series relation to the first switch.

11. Apparatus according to claim 1 wherein the conranged so that saidmaximum force is sufficient to produce burnishing of the conductoragainst the conductive element engaged therewith and insufiicient toproduce appreciable deformation of the wire.

12. Apparatus for welding together work comprised of a wire conductorsurrounded by a layer of pressure deformable insulating material and aconductive element such as a metal terminal pin, the apparatuscomprising (a) a base,

(b) a first welding electrode mounted to the base to be essentiallystationary relative to the base and having a work engaging end,

(c) a second Welding electrode having a Work engaging end and defining apassage therealong from the work engaging end thereof for guiding theinsulated conductor along the second electrode to between the electrodework engaging ends,

(d) means mounting the second electrode in alignment with the firstelectrode and for guided movement relative to the base toward and awayfrom the first electrode from a base position of the second electrode toapply increasing and then decreasing force to work disposed between theelectrodes,

(e) sensing means operative at a predetermined point in movement of thesecond electrode to its base position from adjacent the first electrodefor initiating flow of a pulse of welding current through the electrodesand Work engaged therebetween, and

(f) power means coupled to the second electrode for moving the secondelectrode in said guided movement and arranged to cause the electrodesto exert upon work engaged therebetween a maximum force sufficient toproduce cold flow of the conductor insulating material adequate toestablish a conductive path between the electrodes through the work, thepower means also being arranged so that during the continuance of thecurrent pulse and solidification of molten metal produced by the pulsethe force applied to the work by the electrodes decreases to a value notmore than about 40 percent less than the force applied to the work atsaid predetermined point.

References Cited UNITED STATES PATENTS 2,996,603 8/1961 Stolz et a1.21986 X 3,252,203 5/1966 Alberts et a1. 2l986 X 3,342,972 9/1967 Penberg21986 X 3,436,513 1/1969 Harris 219--86 X JOSEPH V. TRUHE, PrimaryExaminer H. D. JAEGER, Assistant Examiner U.S. Cl. X.R.

ductor is a wire conductor and the power means is ar- 219 '92

